With the recent development of information and communications industry, electronic equipment has been manufactured in smaller, lighter, thinner and more portable fashions. Accordingly, there is an increasing demand for technology of high energy density for batteries used as power sources of such electronic equipment. Lithium secondary batteries are batteries that can best meet such requirements, and thus is being actively researched.
A carbonaceous material has been used as a negative electrode material for lithium secondary batteries. The carbonaceous material includes crystalline carbon and amorphous carbon. The crystalline carbon representatively includes graphite carbon such as natural graphite, and artificial graphite, and the amorphous carbon includes hard carbon (i.e., non-graphitizable carbon) obtained by carbonizing a polymeric resin, soft carbon (i.e., graphitizable carbon) obtained by thermally treating pitch, etc.
In general, soft carbon is made by applying heat having a temperature of approximately 1,000° C. to coke that is a by-product generated during a process of refining crude oil. Unlike conventional graphite-based negative electrode active materials or hard carbon-based negative electrode active materials, the soft carbon has high power and a short period of time required for charging. However, when the soft carbon is used in lithium secondary batteries, an increase in irreversible capacity may be caused, resulting in poor initial efficiency and low reversible capacity.
Such an increase in irreversible capacity is caused when an electrolyte decomposes on a surface of an electrode during charging in order to form a solid electrolyte interphase (SEI) layer as a surface film, and when lithium ions stored in carbon particles during charging are not released during discharging. Among these, the former is a more serious problem, and is known to be a main irreversible cause in forming the surface film.
Accordingly, there is a demand for developing negative electrode active materials capable of replacing conventional negative electrode active materials and improving all discharging capacity, efficiency, and output characteristics when applied to lithium secondary batteries.